Your thread title refers to Windows 7 boot screen? But you are asking for help in Windows 10?
Mike has replied correctly, as to customising the boot screen, but I am thinking you mean the login screen?
In Windows 10, select settings - personalisation, then, on the left, "Lock screen"
In the windows that follows, scroll down and select "Show lock screen background picture on the sign in screen"
Go back up to "Browse and select a picture there, or on your computer.